For a wide variety of military and civilian aviation scenarios air-to-air passive ranging provides a useful way to determine distances between aircraft. The goal of air-to-air passive ranging is to determine the range from one aircraft, called "ownship," to another, called "target", by detecting energy emanating from the target. For this purpose, "ownship" may be equipped with directional receivers that measure the angle of the arriving energy during a data collection interval. To this are added fixes of "ownship's" position and heading obtained from an inertial navigation system (INS) on-board ownship. For most applications, the energy will be in the radio-frequency (RF) or infrared (IR) regions of the electromagnetic spectrum.
A RF system may monitor radar transmissions and RF communications from the target for these purposes. Very long detection ranges are feasible that substantially exceed those at which target could acquire ownship on its own radar. Thus, ownship may be able to locate target without itself being detectable. In this situation, the primary advantage of passive ranging is gained: stealth. Long acquisition range also affords ownship more time to detect target's presence. The directional receiver may be implemented by a two-axis RF interferometer or several such units covering different fields of view.
The infrared system would sense black body radiation emanated from the target. Therefore, operation does not depend on target transmission protocol. Detection range is considerably reduced and ownship would be within range of target's radar. Two factors mitigate this risk: (1) the target might restrict radar usage to avoid detection and (2) convergence time is considerably less for IR passive ranging systems. The latter is attributable to the shorter operating ranges and the higher spatial resolution of the IR sensor. The directional receiver may be implemented using a FLIR imager and video tracker.
A principal deficiency in all air-to-air passive ranging method is ill-conditioning. This is a condition in which small errors in the measurements can cause much larger ones in the computed ranges. There are two principal causes of ill-conditioning: (1) a limited baseline for ranging data; and (2) the need to infer target motion from the data. The relation between baseline ranging accuracy may be explained in terms of triangulation. This is appropriate, although there is no explicit triangulation step in the ranging algorithm, since a triangulation principle is at work whenever range is estimated from sightings at different locations. Factors affecting triangulation accuracy will have similar effects on ranging accuracy. Two of these are the lengths and direction of the baseline ranging data.
Accordingly, there is a need for a method and system that avoids ill-conditioning in passive air-to-air ranging by overcoming the limited ranging data baseline and target motion inference problems of the prior art.